1. Field of the Invention
The present invention relates to a combined oil ring for oil control which is mounted on pistons in reciprocating internal combustion engines and compressors, etc.
2. Description of the Related Art
Two-piece type, steel combined oil rings comprised of an oil ring and an expander are used in internal combustion engines. The combined oil ring is often used in diesel engines. Recently however, because of their excellent durability and low rate of drop in tension due to wear, combined oil rings are also often being used in gasoline engines. The demand in recent years for low fuel consumption has also led to demands for low friction even in oil rings. Measures taken to reduce friction in oil rings include lowering the tension and low-friction surface treating of the outer circumferential surface and upper and lower surfaces.
The oil ring 5A of the two-piece type combined oil ring 4A, as shown in FIGS. 6A and 6B, has upper and lower rails 7A and 8A linked by a web 9. Numerous oil holes 22 are formed in the web 9 to allow oil scraped up by the rails to pass through. An outer circumferential protrusion 10A of the upper rail 7A is formed in a roughly trapezoidal shaped cross section defined by an outer circumferential surface 11A forming a flat surface and upper and lower surfaces 12A and 13A forming a pair of oblique surfaces joining to the surface 11A. The outer circumferential surface 11A and upper and lower surfaces 12A and 13A form sharp corners. The lower rail 8A is formed identical to the upper rail 7A. The reference numeral 1 denotes a piston, 2 denotes a cylinder, 3 denotes a ring groove and 6 denotes a coil expander.
A combined oil ring was disclosed in the prior art for reducing lubricating oil consumption and improving scuff resistance by forming the rail outer circumferential surface as an arc surface whose highest point is located at the center of the axial rail width (See Japanese Utility Model Laid-Open No.4-95169).
A combined oil ring was disclosed in the prior art for limiting the oil scraping effect when the piston slide upwards, by forming the rail outer circumferential surface as a flat surface and a tapered surface (See Japanese Patent Laid-Open No. 9-144881).
The above described combined oil rings of the prior art had the following problems during sliding axial movement in the cylinder while mounted in the ring groove of a piston. The combined oil ring of the prior art formed with only a flat surface as the rail outer circumferential surface had the problem that friction sometimes increased because the sharp corners on the rail outer circumference contacted the inner circumferential surface of the cylinder at localized portions due to piston secondary motion. The combined oil ring of the prior art formed with an arc surface as the rail outer circumferential surface had the problem that contact pressure was too high due to line contact peripherally with the inner circumferential surface of the cylinder and friction sometimes increased. The combined oil ring of the prior art formed with a flat surface and a tapered surface as the rail outer circumferential surface had the problem that the friction sometimes increased due to the lower corner on the lower rail outer circumference during the piston downward stroke. Moreover, when the piston was inclined or rising, the sharp upper corner on the upper rail outer circumference contacted the inner circumferential surface of the cylinder at localized portions and the friction sometimes increased.